The noninvasive nature of MRI has now made it possible to quantify genetic and environmental influences on brain development during childhood and adolescence through the study of pediatric twins. A challenging aspect of large-scale, multicenter or longitudinal MRI studies is avoiding potential biases introduced by differences in equipment at different sites or different times during the study. Measurements such as the volume of subcortical nuclei or the thickness of the cerebral cortex are dependent on both the resolution of the image and the clarity of the contrast between gray matter, white matter, and cerebrospinal fluid, and thus methodologic differences can introduce variation in measurement results (Filippi et al., 1997). For example, in many older MRI studies, brain images were acquired in slices that were 3–5 mm thick. Because 3–5 mm is also the depth of the cerebral cortex, studies done at this resolution would have limited ability to detect changes in cortical thickness, and may even result in different volumes of brain compartments due to partial volume effects at borders between tissue types. More subtle confounds can arise from image
